This invention relates to optical systems used for viewing models of terrain in association with aircraft flight simulation and more particularly with means for reducing the number of mechanical articulations which must be coordinated and servo controlled when multi-stage Scheimpflug correction is provided said systems.
The ability of the Scheimpflug-relay optical probe to produce erect images of near infinite depth of field lends itself to use with aircraft similator training devices where the probe is "flown" over a scale model terrain and the scale factor causes the apparent aperture of the objective lens to be very large, with subsequent reduction in depth of field. The prior art probe consists of a series of lenses, certain of which may be tilted to erect the image and other non-tiltable lenses which are either fixed in position or whose position may be translated along the optical axis of the probe to compensate for changes induced by the tilting lenses, as well as those resulting from variations in altitude and attitude of the probe. If infinite depth of field is to be realistically approached, the movement of the various lenses comprising the probe and the length of the optical path must be controlled precisely.
When an optical probe is used to scan a scale model of the ground in conjunction with aircraft flight-simulation trainers, the optical axis of the probe normally lies along the simulated flight path of the trainer, i.e., above and near parallel to the surface of the scale model being scanned. Of necessity then, the object plane, that of the model, is at an angle to the lens plane of the probe and, as taught by Scheimpflug, the image is formed on a plane inclined to the axis of the lens system of the probe. (See U.S. Pat. No. 751,347 issued Feb. 2, 1904.)
Scheimpflug's early teachings have been extrapolated by latter-day technicians and applied to the problems of (1) erecting the inclined plane of the image; (2) eliminating trapezoidal distortion; while (3) retaining near infinite depth of field in the final image.
The basis for the theoretical design of an optical probe using Scheimpflug correction is the so-called Scheimpflug condition:
The plane containing the original object and the plane containing the projected image, as well as the principal plane of the objective lens, all intersect along one common line defined by Scheimpflug as the axis of collimation. PA1 1. Although the instantaneous field requirement of the dynamic tilting relay lenses is small compared to the total field angle capability, the entire field must be corrected because, as the probe moves from maximum to minimum simulated altitudes, the lenses must tilt through their entire field. Therefore, the correction must be compromised over the entire field rather than being optimized over a smaller instantaneous dynamic field. PA1 2. The entrance and exit pupils and both principal planes of the relay lenses must be coincident at the tilting axis. If they are not, the pupils and image will be displaced off the system axis with tilt. In this case the relay of the image becomes a hopelessly complex design and implementation problem. This requirement severely limits the choice of lens types to a lens which is very difficult to correct without a large number of elements. Poor performance will result due to scattering, absorption and closer tolerance and design constrictions.
Armed with knowledge of the Scheimpflug condition, persons skilled in the prior art of optical probe design have derived the necessary relations the degree of tilt of internal lenses and the variation of optical path length required to meet the problems enumerated above.
However, the complexity of these systems has proven costly and often the mechanical adjustments have been imprecise or precision has been sacrificed to reduce cost.
As an example of the complexity of prior art devices, one may consider that each tilting lens was mounted on an individual shaft which had to be rotated independent of the rotation of other lens shafts. Optical path length adjustments were individually controlled and separate control of the focal length of various lenses had to be incorporated as well. These methods, as already noted, have proven costly and at best imprecise.
A full background discussion of prior art Scheimpflug probes, given in terms of "thin lenses", is presented in U.S. Pat. No. 3,914,011, of R. A. Mecklenborg and R. B. Mallison issued Oct. 21, 1975, which discussion is incorporated herein by reference.
In practice the lens designer's problems are complicated in that the Scheimpflug condition must be modified somewhat for the typical case in which the lens is not a simple thin lens having only one principal plane but is a more complex or "thick lens" having two principal planes. In considering the use of thick lens systems, the axis collimation is considered first to be the line of intersection of object plane with the first principal plane of the lens. This line or axis is then transferred at unit magnification to the second principal plane where it represents the intersection of said second principal plane with the plane of the projected image.
Because a lens has a restricted field of view, there is a practical limit to how much the "Scheimpflug lens" can be rotated. Therefore, there is also a limit to the amount of correction which may be attained using only one such rotating lens. In a probe operating at low altitudes the intermediate image tilt becomes so great that it is not possible to obtain full correction with a single tilting relay and maintain adequate resoltuion. Therefore multiple stages are employed, each reducing the image tilt until the final image is again perpendicular to the system axis. But where lenses are cascaded, light scatter by lens surfaces, transmission losses and tolerance accumulation put a practical restriction on the number of relay stages which may be so used.
The current state-of-the-art has produced a probe with two dynamic tilting Scheimpflug relay lenses and one fixted tilt Scheimpflug relay lens. This system has limited resolution. The limited resolution arises from the fact that in conventional Scheimpflug system there are two requirements which resctrict the relay design possibilities:
It is an object of this invention to simplify the task of the designer of the lens system of the Scheimpflug probe by removing the requirements of total field correction, and zero pupil and principal plane separation imposed on prior art devices.
It is another object of this invention to derive a simple, relatively inexpensive Scheimpflug lens system having high precision which eliminates the need to translate any lenses along the optical axis.
A further objective of the invention is the elimination of the prior art's lens-tilting techniques in favor of a less complex scheme of achieving Scheimpflug correction.